1. Field of the Invention
The present invention relates to a process for the manufacture of pitch foams, to the subsequent conversion of pitch foam to carbon and graphite foam and to improvements in the manufacturing process to enhance the properties of the end products.
2. Description of the Prior Art
This invention deals with carbon in its various forms and, particularly to carbon xe2x80x9cfoamsxe2x80x9d. Carbon foams are a relatively recent area of commercial interest, although carbon fibers have been used commercially in industry for many years. Carbon fibers are known to exhibit extra ordinary mechanical properties due to the unique graphitic morphology of the extruded filaments. Advanced structural composites have been created which advantageously exploit these properties by creating a disconnected network of graphitic filaments held together by an appropriate matrix. Pitch based carbon foams can be considered such an interconnected network of ligaments or struts. As such, pitch based carbon foams represent a potential alternative as a reinforcement in structural composite materials.
Additionally, current applications of carbon fibers have evolved from such structural reinforcement applications to thermal or heat sink applications. For example, heat sinks have been utilized in the aerospace industry to absorb energy in applications such as missiles and aircraft where rapid heat generation is found. A material with a large specific heat capacity is placed in contact with the object that is being heated. During the heating process, heat is transferred to the heat sink from the hot object and, as the heat sink""s temperature rises, it xe2x80x9cstoresxe2x80x9d the heat more rapidly than can be dissipated to the environment through normal convections. Carbon foams have been considered for use as such heat sink materials.
These and other applications have stimulated research into novel reinforcements and composite processing methods for carbon foams. High thermal conductivity, low weight and low coefficient of thermal expansion are of primary concern in such designs. For thermal management applications, certain designs which have been considered included sandwich type approaches in which a low density structural core material, such as a honeycomb or foam, is sandwiched between a high thermal conductivity face sheet. Structural cores of these type are generally limited to low density materials to insure that the weight limits are not exceeded. At the present time, carbon foams and carbon honeycomb materials have generally been the only available materials for use in high temperature applications (exceeding 60xc2x0 C.). High thermal conductivity carbon honeycomb materials have been extremely expensive to manufacture, however, as compared to low conductivity honeycomb materials. Attempts have been made to overcome these shortcomings through the production of pitch based carbon foam materials.
Typical prior art foaming processes utilized a xe2x80x9cblowingxe2x80x9d technique to produce a foam of the pitch precursor. The pitch is melted and pressurized, and then the pressure is reduced. Thermodynamically, this produces a xe2x80x9cFlash,xe2x80x9d thereby causing the low molecular weight compounds in the pitch to vaporize (the pitch boils), resulting in a pitch foam. See Hagar, Joseph W. and Max L. Lake, xe2x80x9cNovel Hybrid Composites Based on Carbon Foams,xe2x80x9d Mat. Res. Soc. Symp., Materials Research Society, 270:29-34 (1992), Hagar, Joseph W. and Max L. Lake, xe2x80x9cFormulation of a Mathematical Process Model Process Model for the Foaming of a Mesophase Carbon Precursor,xe2x80x9d Mat Res. Soc. Symp., Materials Research Society, 270:35-40 (1992), Gibson, L. J. and M. F. Ashby, Cellular Solids: Structures and Properties, Pergamon Press, New York (1988), Gibson, L. J., Mat Sci and Eng A110, 1 (1989), Knippenberg and B. Lersmacher, Phillips Tech. Rev., 36 a (4), (1976), and Bonzom, A., P. Crepaur and E. J. Moutard, U.S. Pat. No. 4,276,246, (1981). Additives can be added to promote, or catalyze, the foaming, such as dissolved gases (like carbon dioxide, or nitrogen), talc powder, freons, or other standard blowing agents used in making polymer foams.
Then, unlike polymer foams, the pitch based foam must generally be oxidatively stabilized by heating in air (or oxygen) for many hours, thereby, cross-linking the structure and xe2x80x9csettingxe2x80x9d the pitch so it does not melt, and deform the structure, during carbonization. See Hagar, Joseph W. and Max L. Lake, xe2x80x9cFormulation for Mathematical Process Model for the Foaming of a Mesophase Carbon Precursor, xe2x80x9d Mat. Res. Soc. Symp., Materials Research Society, 270:35-40 (1992) and White, J. L., and P. M. Shaeffer, Carbon, 27:697 (1989). This is a time consuming step and can be an expensive step depending on the part size and equipment required.
Next, the xe2x80x9csetxe2x80x9d or oxidized pitch foam is then carbonized in an inert atmosphere to temperatures as high as 1100xc2x0 C. Then, a final heat treatment can be performed at temperatures as high as 3000xc2x0 C. to fully convert the structure to carbon and produce a carbon foam suitable for structural reinforcement. The previously described prior art processes resulted in foams which exhibited low thermal conductivities, however.
Other techniques may utilize a polymeric precursor, such as a phenolic, urethane, or blends of these with pitch. See Hagar, Joseph W. and Max L. Lake, xe2x80x9cIdealized Strut Geometries for Open-Celled Foams, xe2x80x9d Mat. Res. Soc. Symp., Materials Research Society, 270:41-46 (1992), Aubert, J. W., (MRS Symposium Proceedings, 207:117-127 (1990), Cowlard F. C. and J. C. Lewis, J. of Mat. Sci., 2:507-512 (1967) and Noda, T., Inagaki and S. Yamada, J. of Non-Crystalline Solids, 1:285-302, (1969). However, these precursors produce a xe2x80x9cglassyxe2x80x9d or vitreous carbon which does not exhibit graphitic structure and, thus, has a very low thermal conductivity and low stiffness as well. See, Hagar, Joseph W. and Max L. Lake, xe2x80x9cIdealized Strut Geometries for Open-Celled Foams, xe2x80x9d Mat. Res. Soc. Symp., Materials Research Society, 270:41-46 (1992).
An improvement to the previously prescribed prior art techniques is described in now issued U.S. Pat. No. 6,033,506, issued Mar. 7, 2000 to Klett and in issued U.S. Pat. No. 6,037,032, issued Mar. 14, 2000, to Klett et al. The process described in these later patents is less time consuming than the techniques previously described, thereby lowering production and fabrication costs. Perhaps more importantly, the Klett process is unique in providing carbon foams with high thermal conductivities, generally greater than 58 W/mK.
Although the Klett process was an improvement in pitch based carbon foaming processes, the Klett process utilized a static pressure during the formation of the green artifact (billet). Routinely, this static pressure selected was about 1000 psig. Graphite artifacts made in this manner have shown a significant density gradient, generally ranging from about 0.25 g/cc at the top of a production billet to about 0.60 g/cc at the bottom of the billet. Such variations can be undesirable, depending upon the particular end application.
A need exists, therefore, for further improvements in pitch based carbon foams and products produced therefrom in which density gradients are reduced.
A need also exists for such a carbon foam exhibiting reduced pore/bubble sizes within the foam during processing.
A need exists for such a process which prevents or reduces thermally induced stresses in the final product.
A need also exists for an improved process for producing a pitch based carbon foam which allows the foam to set faster and which provides an improved ability to manipulate the viscosity of the material during the process stage in which the material is in the liquid/foaming state.
It is an object of the present invention to provide a pitch based carbon foam having a more uniform density gradient profile, with reduced shrinkage and with less tendency to crack as a finished product.
Density variations in currently produced products are thought to occur between the foaming and solidification steps of the process while the foamed pitch is still in the liquid state. The liquid pitch tends to migrate due to gravity, thereby making the bottom of the production billet denser than the top portion of the billet. The present invention has as one object to slow or stop this migration, thereby improving the density uniformity of the ultimately produced product.
By heating the pitch under an increased pressure, the process temperature can exceed the normal foaming point of the pitch without the pitch actually foaming, i.e., the thermal foaming point is raised. Holding the pitch at such a selected temperature allows the growth of mesophase domains within the pitch, thereby increasing the pitch""s viscosity. Higher viscosities at this point in the process reduce the previously described migration problems. The higher pressure of, for example, 8000 psig can then be reduced to, for example, 1000 psig, allowing the heat-treated pitch to foam. Thereafter, a small increase in temperature will solidify the foam. Increasing the viscosity of the pitch also reduces the pore/bubble size of the foam. By manipulating the final process pressure, greater control over pore size is maintained. By changing the hold times and temperatures along with various upper and lower pressure limits, a wider variety of foam products can be produced.
In a specifically preferred process of the invention for producing carbon foam, a pitch precursor is introduced to an appropriate level within a mold. The pitch has a characteristic boiling or foaming point at a given pressure and for a given temperature. Air is purged from the mold and the pitch is pressurized to a preselected initial pressure which will be greater than the ultimate or final pressure utilized. The preselected initial pressure serves to increase the boiling or foaming point of the pitch above the foaming point at the final pressure. The pitch is heated to a temperature below the solidification point but above the liquid and foaming point which typically occurs at the final pressure. The pitch is then depressurized from the initial pressure to the final pressure while maintaining the process temperature above the typical boiling or foaming temperature at the final pressure. The foamed pitch is then heated to a temperature that solidifies the foamed pitch. The solidified foam pitch can then be cooled to room temperature while allowing natural depressurization during cooling to thereby produce a carbon foam.
Additional objects, features and advantages will be apparent in the written description which follows.